The present invention relates to a noise reduction system for a passenger compartment of an automotive vehicle by positively generating the sound to cancel the vehicle internal noise.
There have been proposed several techniques for reducing the noise sound generated mainly from an engine and transmitted to the passenger compartment by producing canceling sound from a sound source disposed in the passenger compartment. The amplitude of the canceling sound is the same as that of the noise sound, but the canceling sound has a reversed phase to that of noise sound.
As a recent example, Japanese application laid open No. 1991-178845 discloses a vehicle internal noise reduction technique for reducing a noise sound by using a LMS (Least Means Square) algorithm (a theory for obtaining a filter coefficient by approximating it to a means square error in order to simplify a formula to obtain a filter coefficient: noticing that a filter correction formula is a recursive expression) or by employing a MEFX-LMS (Multiple Error Filtered X-LMS) algorithm (a multi-channeled algorithm of the LMS algorithm). This technique has already been put to practical use in some of vehicles. In an internal noise reduction system using this LMS algorithm, when an internal noise whose primary source is an engine vibration is reduced, a vibration noise source (hereinafter referred to as a primary source, too) is obtained from a signal having a high correlation with an engine vibration. The canceling sound is produced from a speaker after the primary source is synthesized by an optimum filter. Then a reduced sound is detected by a microphone as an error signal. A filter coefficient of the optimum filter is updated by the LMS algorithm based on this error signal and the above primary source so as to optimize the reduced sound at a sound receiving point.
In this prior art technology, however, since in the noise reduction system using the LMS algorithm or the MEFX-LMS algorithm abovementioned the error signal detected by the microphone contains other noise components (random noise signals) than those to be reduced, the filter coefficients are updated being affected by these random signals. As a result of this, the calculation amount in converging the filter coefficients is increased, so that not only a response characteristic becomes poor because the noise control efficiency get worsened, but also an obtainable amount of noise reduction can not be secured because the noise reduction control becomes unstable due to the effects of those random noise signals.
In view of the foregoing, it is an object of the present invention to provide a vehicle internal noise reduction system which has an excellent response characteristic and a good noise reduction performance.
To achieve the abovementioned object, the vehicle internal noise reduction system according to the present invention comprises: input signal transforming means responsive to an ignition pulse signal for transforming the ignition pulse signal into a single vibration noise source signal (primary source) so as to obtain a frequency spectrum composed of 0.5.times.n order components; canceling signal synthesizing means responsive to the vibration noise source signal for synthesizing the transformed vibration noise source signal into a canceling signal based on filter coefficients of an adaptive filter and for outputting the synthesized canceling signal; canceling sound generating means responsive to the synthesized canceling signal for generating a canceling sound to cancel a vibration noise in a passenger compartment of a vehicle; error signal detecting means for detecting a reduced sound as an error signal at a noise receiving point; noise components compressing means responsive to the vibration noise source signal for compressing the error signal so as to reduce an influence of random noise components other than noises to be reduced contained in the error signal; and coefficients updating means responsive to the vibration noise source signal and the compressed error signal for updating the filter coefficients of the adaptive filter.
Next, based on the composition of means abovementioned, it will be briefly explained how the noise reduction system according to the present invention functions.
First, when a vibration noise originated from an engine is transmitted to the passenger compartment, the vibration noise source signal is synthesized into a canceling signal by the adaptive filter. Then, this canceling signal is transformed into a canceling sound by the canceling sound generating means and the canceling sound is generated from a sound source to cancel the vibration noise. Further, a reduced sound is detected by the error signal detecting means as an error signal. Next, noise components containing in the above error signal are compressed up to a specified level by the noise components compressing means based on the above vibration noise source signal. Finally, based on the vibration noise source signal and the compressed noise components, filter coefficients of the adaptive filter are updated by the coefficients updating means.
Thus, since the noise components other than those to be reduced are compressed up to a specified level, a converging performance of the adaptive filter is improved, whereby the noise reduction system according to the present invention can attain a good response characteristic and an excellent noise reduction performance.